Transformation specific proteins induced by mammalian replication-defective transforming RNA tumor viruses continue to be sought. In order to identify these proteins, two different experimental lines have been followed. We have persisted in the isolation of a variety of cloned sarcoma virus infected, replication-defective cell lines expressing viral-coded transforming properties. One result of these studies has been the identification of several mutant cell lines characterized by nonconditional defects in the expression of transformation. The availability of such cell lines should aid in the identification of viral specified transforming proteins. Another effort has involved the placement of a variety of mammalian sarcoma viruses with and without a common helper virus in single cell clones of a cell line derived from an inbred rat colony. These syngeneic cell lines have been used to immunize animals in the inbred colony and the antisera so derived have been employed in identifying transformation specific proteins.